Various ways of reducing the slip of wheels are known in the art. Especially when a wheel driven vehicle is moved on slippery grounds measures have to be taken in order to ensure sufficient driving stability and, thus, safety. In particular, drive wheels of floor cleaning machines—if located behind the scrubbing unit of the machine—face severe traction problems: cleaning agents solved in the cleaning solution create a slippery area which can more or less be compared to an icy surface. Generally, the measures to reduce wheel slip consist in increasing the wheel traction by providing a tread profile which supports better friction. Several approaches to attack this problem are known wherein the tread profile can have various forms.
One possible approach consists in providing small separate recesses hollows in the outer surface of a conventional wheel in order to create a suction effect when such a recess or hollow contacts the ground. This approach has been known for quite some time, and DE 464 198, DE 641 896, DE 1 004 505, DE 2 313 29, and DE 42 06 133 are early and recent examples thereof. DE 199 52 878 aims at improving this suction effect by providing small channels inside the wheel connecting the hollows at the outer surface thereof with a device for creating a vacuum. However, the wheels according to these documents have the common drawback that the recesses or hollows are located at the outer surface of the wheel only (like a regular tread profile) and thus vanish over time due to abrasion. Consequently, as is the case with any conventional tread profile, the wheels become slick with reduced depth of the recesses.
Another approach consists in mixing particles to the wheel material during the manufacturing process wherein the particles drop out of the wheel material when they reach the outer surface of the wheel. WO 00/67976 discloses such a method of dosage and mixing of solid particles to non-vulcanized rubber or other polymeric materials using a stud extruder. This method particularly applies to the addition of abrasive particles to rubber for the production of friction tyres or antiskid soles for shoes. However, this method has the drawback that the solid particles which have been mixed to the wheel material continuously drop out during use of the wheels, thus leaving residues on the ground. Furthermore, a certain grinding effect is generated due to the solid particles which have not yet dropped out and which still project from the wheel surface. Whereas the residues and the grinding effect do not pose any problems for outdoor use, e.g., of a conventional car, they are counterproductive when the purpose of the wheel driven vehicle consists in cleaning the floor on which it runs.
Apart from the fact that the wheels of floor cleaning machines should not produce additional residues on the floor, care must also be taken that the wheels are non-marking—for example, a black wheel with coal as a filler is not feasible. Thus, one approach is based on rubber wheels with chalk as a filler (grey wheels). However, these wheels only deliver traction for a short period of time. Another solution is based on a special rubber (Vulkollan). However, like in the former case, the traction decreases over time, and in addition to that, the manufacturing process is complicated and the base material is expensive. Still another approach consists in using polyurethane as the wheel material. However, this material is also very expensive and has the additional drawback that it is very rigid which in turn makes even smallest unevenesses noticeable. Thus, a modified approach consists in providing wheels with a rubber core and an outer surface made of polyurethane which results in better damping at even lower costs. However, using polyurethane—even with a conventional tread profile—for the wheel still does not lead to proper traction when it runs on a ground which is covered with a cleaning solution.